Currently available robot arm mechanisms include pivotally joined multiple links that are driven by a first motor and are mechanically coupled to effect straight line movement of an end effector or hand and are equipped with a second motor to angularly displace the hand about a central axis. Certain robot arm mechanisms are equipped with telescoping mechanisms that move the hand also in a direction perpendicular to the plane of straight line movement and angular displacement of the hand. The hand is provided with a vacuum outlet that secures a specimen, such as a semiconductor wafer, computer hard disk, or compact disk, to the hand as it transports the specimen between processing stations.
U.S. Pat. No. 4,897,015 of Abbe et al. describes a rotary-to-linear motion robot arm that uses a first motor to control a multiple link robot arm to produce straight line radial motion from motor-driven rotary motion. An additional motor may be coupled to the robot arm for operation independent of that of the first motor to angularly move the multiple link robot arm without radial motion. Because they independently produce radial motion and angular motion, the first and second motors produce useful robot arm movement when either one of them is operating.
The robot arm of the Abbe et al. patent extends and retracts an end effector (or a hand) along a straight line path by means of a mechanism that pivotally couples in a fixed relationship a first arm (or forearm) and a second (or upper) arm so that they move in predetermined directions in response to rotation of the upper arm. To achieve angular displacement of the hand, a third drive motor rotates the entire robot arm structure. The Abbe et al. patent describes no capability of the robot arm to reach around corners or travel along any path other than a straight line or a circular segment defined by a fixed radius.
U.S. Pat. No. 5,007,784 of Genov et al. describes a robot arm with an end effector structure that has two oppositely extending hands, each of which is capable of picking up and transporting a specimen. The end effector structure has a central portion that is centrally pivotally mounted about the distal end of a second link or forearm. The extent of pivotal movement about all pivot axes is purposefully limited to prevent damage to vacuum pressure flexible conduits resulting from kinking or twisting caused by over-rotation in a single direction.
The coupling mechanism of a first link or upper arm, the forearm, and the end effector structure of the robot arm of the Genov et al. patent is more complex than that of the robot arm of the Abbe et al. patent. Nevertheless, the robot arm structures of the Abbe et al. and Genov et al. patents operate similarly in that each of the end effector structures picks up and transports specimens by using one motor to extend and retract a hand and another, different motor to rotate the entire robot arm structure to allow the hand to extend and retract at different ones of a restricted number of angular positions.
Robot arms of the type described by the Abbe et al. and Genov et al. patents secure a specimen to the hand by means of vacuum pressure delivered to the hand through fluid conduits extending through the upper arm, forearm, and hand and around all of the pivot axes. The Abbe et al. patent is silent about a vacuum pressure delivery system, and the Genov et al. patent describes the use of flexible fluid conduits. The presence of flexible fluid conduits limits robot arm travel path planning because unidirectional robot arm link rotation about the pivot axes "winds up" the conduits and eventually causes them to break. Thus, conduit breakage prevention requirements prohibit continuous robot arm rotation about any of the pivot axes and necessitate rewind maneuvers and travel path "lockout" spaces as part of robot arm travel path planning. The consequences of such rewind maneuvers are more complex and limited travel path planning, reduced throughput resulting from rewind time, and reduced available work space because of the lockout spaces.
Moreover, subject to lockout space constraints, commercial embodiments of such robot arms have delivered specimens to and retrieve specimens from stations angularly positioned about paths defined only by radial distances from the axes of rotation of the robot arms.
Thus, the robot arm structures described by the Abbe et al. and Genov et al. patents are incapable of transporting specimens between processing stations positioned in compact, irregularly shaped work spaces. For example, neither of these robot arm structures is set up to remove or place specimen wafers in wafer cassettes that have openings positioned side-by-side in a straight line arrangement of a tightly packed work space.
A solution to the above-described problems is described in allowed U.S. patent application Ser. No. 5,765,444, filed Jul. 10, 1995, for DUAL END EFFECTOR, MULTIPLE LINK ROBOT ARM SYSTEM WITH CORNER REACHAROUND AND EXTENDED REACH CAPABILITIES, which is assigned to the assignee of this application and is incorporated herein by reference. A multiple link robot arm mechanism is mounted at the distal end of a torso link that is capable of 360-degree rotation about a central, or "torso," axis. Two coaxially arranged motors mounted at the distal end of the torso link are capable of synchronized operation that moves a robot arm hand along a curvilinear path as the extension of the hand changes. A first motor rotates a forearm about an elbow axis that extends through distal and proximal ends of the upper arm and forearm, respectively, and a second motor rotates an upper arm about a shoulder axis that extends through a proximal end of the upper arm. A mechanical linkage couples the upper arm and the forearm. The mechanical linkage forms an active drive link and a passive drive link. The active drive link operatively connects the first motor and the forearm to cause the forearm to rotate about the elbow axis in response to the first motor. The passive drive link operatively connects the forearm and the hand to cause the hand to rotate about a wrist axis in response to rotation of the forearm about the elbow axis. The wrist axis extends through distal and proximal ends of the forearm and hand, respectively.
Whenever the first and second motors move equal angular distances, the angular displacement of the upper arm about the shoulder axis and the angular displacement of the forearm about the elbow axis equally offset and thereby result in only a net angular displacement of the hand about the shoulder axis. Thus, under these conditions, there is no linear displacement of the hand and no rotation of the hand about the wrist axis. Whenever the first and second motors move different angular distances, the angular displacement of the upper arm about the shoulder axis and the angular displacement of the forearm about the elbow axis only partly offset and thereby result in angular displacements of the hand about the shoulder and wrist axes and a linear displacement of the hand. Accordingly, coordination of the position control of the first and second motors enables the robot arm mechanism to describe a compound curvilinear path of travel for the hand.
A third, or torso, motor rotates the torso link about the central axis to permit rotation of the torso link independent of the motion of the robot arm mechanism mounted to it. The presence of the rotatable torso link together with the independent robot arm motion permits simple, nonradial positioning of specimen processing stations relative to the torso axis, extended paddle reach, and corner reacharound capabilities. Moreover, the rotating joints of the robot arm mechanism include rotary vacuum slip rings that permit continuous rotation about the shoulder, elbow, and wrist axes without a need to unwind to prevent kinking or twisting of vacuum lines.
Unfortunately, mounting the first and second motors at the distal end of the torso link creates a relatively high moment of inertia for the torso motor to work against. The moment of inertia may be reduced by making the first and second motors as small and as light as possible, but this limits their torque and the rate at which the robot arm can rotationally move specimens.
What is needed, therefore, is a continuously rotatable robot arm mechanism that is capable of accurately and rapidly moving large specimens along compound curvilinear paths.